cualquier ayuda será bienvenida

Si pongo master me sale esto:

Problem 1
- The requested package doctrine/doctrine-bundle (locked at v1.2.0, required as >1.6) is satisfiable by doctrine/doctrine-bundle[v1.2.0] but these conflict with your requirements or minimum-stability.
Problem 2
- The requested package twig/extensions (locked at v1.0.0, required as >1.0) is satisfiable by twig/extensions[v1.0.0] but these conflict with your requirements or minimum-stability.
Problem 3
- The requested package symfony/assetic-bundle (locked at v2.3.0, required as >2.8) is satisfiable by symfony/assetic-bundle[v2.3.0] but these conflict with your requirements or minimum-stability.
Problem 4
- The requested package symfony/monolog-bundle (locked at v2.3.0, required as >2.3) is satisfiable by symfony/monolog-bundle[v2.3.0] but these conflict with your requirements or minimum-stability.
Problem 5
- The requested package incenteev/composer-parameter-handler (locked at v2.0.0, required as >2.0) is satisfiable by incenteev/composer-parameter-handler[v2.0.0] but these conflict with your requirements or minimum-stability.
Problem 6
- Installation request for symfony/symfony master -> satisfiable by symfony/symfony[dev-master].
- symfony/symfony dev-master requires php ^7.1.3 -> your PHP version (5.5.9) does not satisfy that requirement.